1. Technical Field to Which the Invention Pertains
The present invention relates to a light emitting element using a semiconductor and to a method for fabricating the same.
2. Prior Art
The use of a nitride-based compound semiconductor represented by AlInGaN has enabled the commercialization of light emitting elements which output light at the ultraviolet, blue, and green wavelengths from which it has heretofore been difficult to obtain a sufficient emission intensity, such as a light emitting diode (LED) and a semiconductor laser, so that research and development thereof has been conducted vigorously. Among light-emitting elements, an LED is easier to fabricate and control than a semiconductor laser and longer in lifespan than a fluorescent lamp so that the LED, particularly using a nitride-based compound semiconductor, is considered to be promising as a light source for illumination.
FIG. 35 is a perspective view showing a conventional nitride-based compound semiconductor LED. The conventional LED has a structure in which an n-type GaN layer 1002, an InGaN active layer 1003, and a p-type GaN layer 1004 are formed successively through crystal growth on a sapphire substrate 1001. Each of the InGaN active layer 1003 and the p-type GaN layer 1004 has been removed partly by etching so that the n-type GaN layer 1002 is exposed. An n-side electrode 1006 is formed on the exposed portion of the n-type GaN layer 1002. A p-side bonding electrode 1007 is provided on the p-type GaN layer 1004.
The following is the operation of the LED.
First, holes injected from the p-side bonding electrode 1007 are diffused laterally in a p-side transparent electrode 1005 to be injected from the p-type GaN layer 1004 into the InGaN active layer 1003.
On the other hand, electrons injected from the n-side electrode 1006 are further injected into the InGaN active layer 1003 through the n-type GaN layer 1002. The recombination of the holes and the electrons in the InGaN active layer 1003 causes light emission. The light is emitted to the outside of the LED through the p-side transparent electrode 1005.
However, it cannot be said that such a conventional structure has sufficiently high light extraction efficiency. The light extraction efficiency is the ratio of light generated in the active layer and emitted from the LED into an air to all the light generated in the active layer. The cause of the low light extraction efficiency of the conventional LED is the refractive index of a semiconductor which is higher than that of the air so that the light from the active layer is totally reflected by the interface between the semiconductor and the air and confined to the inside of the LED. For example, the refractive index of GaN is about 2.45 at a wavelength of 480 nm so that a critical angle of refraction at which total reflection occurs is as small as about 23 degrees. That is, the light emitted from the active layer at an angle larger than the critical refraction angle in terms of a normal to the interface between the semiconductor and the air is totally reflected by the interface between the semiconductor and the air so that the light emitted from the active layer and extractable to the outside of the LED accounts for only about 4% of all the light emitted from the active layer. Accordingly, the problem is encountered that external quantum efficiency (the ratio of light that can be extracted from the LED to currents supplied to the LED) is low and power conversion efficiency (the ratio of a light output that can be produced to all the supplied power) is lower than that of a fluorescent lamp.
As a solution to the problem, a technology which forms a photonic crystal at the surface of the LED has been proposed, as disclosed in Japanese Laid-Open Patent Publication No. 2000-196152.
FIG. 36 is a perspective view showing a conventional LED having an upper surface formed with a photonic crystal. As shown in the drawing, two-dimensional periodic projections/depressions are formed in the p-type GaN layer 1004 according to the conventional embodiment. In the structure, even light emitted from the active layer at an angle larger than the critical refraction angle in terms of the normal to the interface between the semiconductor and the air can have the direction of emission at an angle smaller than the critical refraction angle due to diffraction by the periodic projection/depressions. This increases the ratio of light emitted to the outside of the LED without being totally reflected and improves the light extraction efficiency. In the present specification, the wording “two-dimensional periodic” or “two-dimensional” may indicate that structures are formed to have given spacings (a given period) along a first direction in a plane, while they are also formed to have given spacings (a given period) along a second direction crossing the first direction. For example, “two-dimensional periodic projections” means a plurality of projections which are periodically arranged along two directions in a plane as shown in, for example, FIG. 36.